Structural and dynamical comparison of α, β and γ forms of murine epidermal growth factor

Reversed‐phase high‐performance liquid chromatography of mouse epidermal growth factor (mEGF) purified by the method of Savage and Cohen [1] allows resolution of four forms of the protein hormone: α, β, γ and δ. α‐mEGF, the major form isolated by HPLC, is the parent mEGF originally sequenced by Savage and Cohen [1], and β‐mEGF is des‐asparaginyl¹‐α‐mEGF. Proton nuclear magnetic resonance spectroscopy has been used to investigate structural and dynamical differences among the α, β and γ forms of the peptide. Based on these data, γ‐mEGF can be tentatively identified as des‐Asn¹, Ser²⁻mEGF. Comparative nuclear Overhauser experiments on amide and aromatic proton resonances suggest that there are significant conformational changes in the peptide structure on cleavage of the N‐terminal residues. Backbone amide proton/deuteron exchange rates in γ‐mEGF and β‐mEGF are significantly faster than those in α‐mEGF suggesting that structural dynamics are enhanced in the minor forms; this interpretation is supported by the decrease in Tyr(2,6)–(3,5) intraresidue NOE magnitudes on going from α to β to γ forms. These data suggest that the average conformations of β and γ‐mEGF favor a more open or denatured state of the protein and that the N terminus is critical to the structural integrity of the parent protein.